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Citrus is a highly nutritive and prized fruit crop around the world. It contributes a substantial share in local consumption and exports of a nation to earn a handsome foreign exchange. The production of citrus is under the threat of citrus decline. Different factors are responsible for the citrus decline but virus and virus-like diseases have the major role in this decline. Virus and virus-like diseases alone or in association with other biotic and abiotic factors exist in the citrus orchards. Therefore, indexing of diseases caused by virus and virus-like pathogens is the key factor to manage these citrus diseases. Proper facilities and skilled personnel are the pre-requisite for the diseases indexing procedures. Biological, serological and molecular indexing is sensitive, reliable and durable strategy for managing different citrus virus and virus-like diseases under different conditions. Moreover, indexing of viruses and virus-like pathogens are very important for the production of disease free citrus nurseries. This chapter gives a brief review for the commonly used biological, serological and molecular assays for the detection of citrus virus and virus-like pathogens.

Collecting, conservation and utiklization of plant genetic resources and their global distribution are essential componentes of international crop improvement programmes. Inevitably, the movement of germplasm involves a risk of accidentally introducing plant quarantine pests*along with the host plant material; in particular, pathogens that are often symptomless, such as viruses, pose a special risk. In order to minimize this risk, effective testing (indexing) procedures are required toensure that distributed material is free of pests that are of quarantine concern. The ever-incrrrreasing volume of germplasm exchanged internationally, coupled with recent rapid advances in biotechnology, has created a pressing need for crop-specific overviews of the existing kno.

Diseases caused by viruses and mycoplasmalike bodies are among the greatest threats to citrus. Viruses consist of either ribonucleic acid (RNA) or deoxyribonucleic acid (DNA), usually surroundedby a very thin protein coat. Mycoplasmas resemble small "L" (irregularly shaped) forms of certain bacteria, and are thousends of times larger than viruses. Mycoplasmas contain both RNA and DNA, and have a complex internal structure, including ribosomes, all enclosed within a membrane. Mycoplasmas reproduce themselves, whereas virus replication is an activity of the infected host under the influence of the virus. Viruses are incapable of reproducing themselves in vitro. Recent evidence suggests that stubborn disease and greening disease, long considered to be caused by viruses, may instead be caused by mycoplasmalike bodies.